With increasing recognition of environmental protection, environmental regulations are gradually becoming stricter. It is believed that decreasing the sulfur content in gasoline or diesel fuel is one of the most important measures to improve the air quality because the sulfur contained in the fuel adversely affects the performance of the catalytic converter of automobiles and vehicles. The sulfur oxide present in the exhaust gas from automobile engine inhibits the activity of the noble metal catalyst in the converter and poisons the catalyst irreversibly. The gases released from the ineffective or poisoned converter comprise uncombusted non-methane hydrocarbons, nitrogen oxide and carbon monoxide, all of which easily form photochemical smogs when catalyzed by sunlight.
In China, most sulfur contained in gasolines comes from heat processed gasoline, which is mainly catalytic cracking gasoline. Therefore, decreasing the sulfur content in the cracking gasoline would facilitate reducing sulfur content of these gasolines. The current standard for gasoline product is GB 17930-2006 “Motor Vehicle Gasoline,” which further restricts the sulfur content of gasoline and requires that by Dec. 31, 2009 the sulfur content of gasoline should be lowered to 50 ppm. This circumstance means that cracking gasoline must be desulfurized to a great degree in order to meet the environmental requirement.
When lowering the sulfur content of motor vehicle fuel, changes in olefin content which leads to a reduction of octane number (including Research Octane Number, ROM and Motor Octane Number, MON) should be avoided so to retain the combustion characteristics of the motor vehicle fuel. Generally, the negative change on the olefin content is caused by the hydrogenation reaction induced upon removal of thiophene compounds (including thiophene, benzothiophene, alkylthiophene, alkylbenzothiophene and alkyldibenzothiophene). Further, the loss of aromatic hydrocarbons in the cracking gasoline due to saturation under hydrogenation condition should also be avoided. Therefore, the most desirable approach is to desulfurize the gasoline while retain its octane number.
On the other hand, both hydrodesulfurization and hydrogenation of hydrocarbons consume hydrogen, thereby increasing the operation cost of the desulfurization. Accordingly, there is a need for a method of desulfurization without consuming large volume of hydrogen, thereby providing a more economical method for treatment of cracking gasoline or diesel fuel.
Traditionally, a fixed-bed process is usually used for desulfurization in a liquid phase. However, this process is disadvantageous in the homogeneity of reaction and the regeneration of the material. Compared with the fixed-bed process, a fluidized-bed process is advantageous given wider applications prospects in the future because of better heat transfer and pressure drop. In this context, a fluidized-bed reactor is usually provided with granular reactants; and the catalyst particulate used needs sufficient abrasion-resistance. Accordingly, it is of great significance to find an adsorbent with both excellent abrasion-resistance and desulfurization performance.
Chinese Patents CN 1110931A and CN 1151333A describe a new absorption composition comprising zinc oxide, silicon dioxide, colloidal oxide and an accelerant; and a process for making the same. In the process, fluidizable particles are produced by a pressure forming technique, and the particle pore volumes are increased by adding to the colloid a pore-forming agent which becomes flammable when heated.
U.S. Pat. No. 6,150,300, Chinese Patent publications CN 1355727A and CN 1382071A disclose a granular adsorbent composition comprising a mixture of zinc oxide, silica, alumina, nickel or cobalt in a reduced state. The adsorbent is made by first mixing silica, alumina and zinc oxide under shearing force, and preparing the solid particle through a granulating machine. In these methods the active component nickel is introduced by impregnation.
Chinese Patent publications CN1422177A and CN 1627988A disclose an abrasion resistant adsorbent composition prepared by impregnating an adsorbent carrier comprising zinc oxide, expanded perlite and alumina with an accelerant such as nickel, nickel oxide, or with a precursor of nickel oxide, and subsequently reducing the valence of the accelerant metal in the resulting accelerant metal adsorbent carrier composition. The adsorbent composition is useful for removing elemental sulfur and sulfide, for example hydrogen sulfide and organic sulfide, from cracking gasoline and diesel fuel. According to these patents, the active component can be introduced by impregnating the adsorbent carrier with an accelerant metal containing precursor, which impregnation method is advantageous in easy preparation and implementation. However, the accelerant metal introduced thereby often does not have sufficient homogeneity, which can lead to insufficient desulfurizing activity in the adsorbent. In this situation, the amount of accelerant metal used is typically increased to enhance the activity of the adsorbent, but which also increases the cost of making the adsorbent.